Javascript(How to check Session)

function CheckSession() {

                var session = '<%=Session["sessionName"] != null%>';

                //session = '<%=Session["sessionName"]%>';

                alert(session);

                if (session == false) {

                    alert("Your Session has expired");

                    window.location = "login.aspx";

                }

                else {

                    alert(session);

                }

            }